Aerial booms are used on a variety of machines which operate to raise and lower heavy loads. Examples include cranes, digger derrick machines and aerial devices which have materials handling capability. By way of example, a vehicle mounted aerial device typically includes a boom which can rotate and pivot up and down. The boom assembly normally includes a lower boom mounted to the vehicle and an upper boom which articulates relative to the lower boom. The tip of the upper boom carries a working platform which may be a bucket or basket from which workers can perform various jobs. It is common for this type of aerial device to be used in materials handling applications involving the raising and lowering of heavy loads. Materials handling is usually carried out by a pivotal jib on the tip of the upper boom and a winch having a line passing over a sheave on the tip of the jib. Electrical transformers and other heavy loads can be raised and lowered on the winch line.
Aerial machines have maximum load ratings that limit the loads that can be lifted at each different boom position. Industry regulations and general safety considerations require that the maximum load rating not be exceeded. Consequently, it is important for the machine operator to know the weight of the load so that he can make certain that he is operating safely within the rated capacity of the machine.
The loads which are handled by aerial devices, cranes and digger derricks can be measured in various ways. The prevalent technique in the past has involved the installation of a tensiometer or similar device to measure the tension in the winch line. Examples of machines which use tensiometers are disclosed in U.S. Pat. No. 3,278,925 to Saunders et al., U.S. Pat. No. 4,003,482 to Cheze, U.S. Pat. No. 4,098,410 to Nixon et al. and U.S. Pat. No. 4,746,024 to Hensler. As exemplified by these patents, the tensiometer is constructed with three sheaves or rollers, two of which maintain the line at a constant wrap angle on the third sheave or roller (the wrap angle being the angular arc along which the line contacts the third sheave or roller). The force component in a selected direction applied to the third sheave or roller is measured by any of a variety of suitable devices. With a constant wrap angle, the line tension can be calculated from the measured force which is applied to the measurement sheave or roller.
Although tensiometers applied in this fashion can provide accurate load measurements, they are not without problems. They require three sheaves or rollers and are thus somewhat complicated mechanically. It is also necessary to provide the tensiometer as a separate component which must be added to the machine and have the winch line threaded through it. Thus, the expense and complexity of the machine are increased, as are maintenance and reliability problems.
One type of commercially available device that can be used to measure a load in a given direction or two mutually perpendicular directions is a load pin. A load pin includes one or more strain gauges which provide electrical output signals proportional to the strain they sense. A dual axis load pin is useful in a situation where the wrap angle varies, so long as either the entry angle or exit angle of the line is constant. However, even a dual axis load pin is unable to measure a load if both the entry angle and the exit angle vary, as is the case with a load bearing sheave on a aerial boom or jib. In this context, the entry angle is the angle of the part of the line extending from the winch to the sheave relative to another axis (such as the longitudinal axis of the jib or boom). The exit angle is the angle of the part of the line extending from the load to the sheave relative to the other axis.
As the boom or jib angle varies from horizontal, the exit angle of the line from the sheave also varies. The entry angle of the winch line varies somewhat with changes in the boom or jib angle. Sometimes, the winch is mounted on the boom so that when the jib is articulated relative to the boom, the entry angle can change significantly. Even if the winch is mounted on the jib, the entry angle can still change. One reason is that the jib deflects to varying degrees when it is handling different loads. Also, the winch line approaches the sheave at a different angle when it is fully wound on the winch drum than in a case where it is fully or nearly fully unwound from the drum.
The situation is even more complicated with an overcenter machine which is one where the boom or jib can move past a vertical position. An overcenter machine is equipped with two load bearing sheaves which are used alternately when the aerial boom or jib is on opposite sides of vertical. In this case, it must be taken into account that either sheave can be loaded when the machine is operating and that the load handled by the boom or jib can apply moments in opposite directions when the structuL,e is on opposite sides of vertical.